\(\bullet \)  Log-normal estimation The idea is to get all eCPM_cost distributions at different bid price \(bid\) , and then use log-normal distribution [1]} to model each eCPM_cost distribution with sample mean \(\mathbb {E}(cost)\)  and variance \(Var(cost)\) . Finally return the \(\mathbb {E}(cost)\)  as the predicted cost for the given bid price.
